A “Deadfire” is a military word meaning the following:
Deadfiring is firing multiple missiles or other projectiles without having a lock on or aiming. If firing in Deadfire mode, you are usually aiming in the general area of the target and firing. Used in high stress situations, as well as a scare tactic.
So in other words you aim to basically to it and everything around it, with massive firepower.
OMG, I owned that Noob with a deadfire of missles